Low Loss Bridge Electrode with Rounded Corners for Electro-optic Modulators

ABSTRACT

An electro-optic modulator having a substrate, one or more optical waveguides, at least one active electrode formed on the substrate and aligned over the optical waveguide, the active electrode operating to induce a refractive index change in the optical waveguide. The active electrode has a lower surface arranged facing the substrate, an upper surface arranged away from the substrate, substantially planar side walls, and rounded corners between the side walls and the lower surface of the active electrode. The electrode can be a bridge electrode, with two lower portions and an upper portion connected to the lower portions, the lower portions spaced apart from each other, each of the two lower portions of the active electrode extending over one of the optical waveguides. Each of the lower portions has rounded convex corners. The upper surface of the electrode can also have rounded corners or a completely rounded upper surface.

BACKGROUND

1. Field of the Invention

This application is related to electro-optic modulators and electrodesfor use in electro-optic modulators.

2. Description of Related Art

Electrooptic devices, such as optical modulators, have the ability tochange a particular characteristic of an optical signal, such as itsintensity, phase, or polarization. Electro-optic modulators,particularly lithium niobate (LiNbO₃) modulators, have application inradio frequency analog links, digital communications and electric fieldsensing. Electro-optic modulators are useful for modulating an opticalsignal in a waveguide with an RF or other frequency electrical signal.

A variety of electro-optic modulators are disclosed in Wooten, E. L,Kissa, K. M., Yi-Yan, A., Murphy, E. J., Lafaw, D. A., Hallemeier, P.F., Maack, D., Attanasio, D. V., Fritz, D. J., McBrien, G. J., Bossi, D.E., “A review of Lithium Niobate Modulators for Fiber-OpticCommunications Systems,” IEEE Journal of Selected Topics in QuantumMechanics, Vol. 6, No. 1, 2000.

Electro-optic modulators formed in x-cut and z-cut lithium niobate aredisclosed in U.S. Pat. No. 5,416,859 to Burns et al., U.S. Pat. No.6,016,198 to Burns et al., U.S. Pat. No. 6,304,685 to Burns, U.S. PatentApplication Publication No. 2004/0061918 A1, U.S. Patent ApplicationPublication No. 2004/0095628 A1, U.S. Patent Application Publication No.2004/0136634 A1, U.S. Patent Application Publication No. 2004/0151414A1, U.S. Pat. No. 5,388,170, U.S. Pat. No. 5,712,935, U.S. Pat. No.6,522,793, U.S. Pat. No. 6,600,843, U.S. Patent Application PublicationNo. 2004/0114845 A1, U.S. Pat. No. 5,153,930, U.S. Pat. No. 5,189,713,U.S. Pat. No. 5,953,466, U.S. Pat. No. 6,501,867, U.S. PatentApplication Publication No. 2004/0066549, U.S. Patent ApplicationPublication No. 2004/0145797, and U.S. Patent Application PublicationNo. 2003/0228081. Electro-optic devices with a lithium niobate substrateare also disclosed in U.S. Patent Application Publication No.2004/0202395, U.S. Patent Application Publication No. 2004/0240036, U.S.Patent Application Publication No. 2004/0240790, U.S. Patent ApplicationPublication No. 2004/0247220, U.S. Patent Application Publication No.2004/0264832, U.S. Pat. No. 5,442,719, U.S. Pat. No. 5,497,233, U.S.Pat. No. 6,128,424, and U.S. Patent Application Publication No.2004/0067021.

Reflection traveling-wave interferometric modulators are disclosed in W.K. Burns, M. M. Howerton, R. P. Moeller, R. W. McElhanon, A. S.Greenblatt, “Broadband reflection traveling-wave LiNbO3 modulator”, OFC'98 Technical Digest, 1998, pp. 284-285, and in W. K. Burns, M. M.Howerton, R. P. Moeller, R. W. McElhanon, A. S. Greenblatt, “ReflectionTraveling Wave LiNbO3 Modulator for Low Vπ Operation,” LEOS 1997, IEEE p60-61, and in W. K. Burns, M. M. Howerton, R. P. Moeller, A. S.Greenblatt, R. W. McElhanon, “Broad-Band Reflection Traveling-WaveLiNbO3 Modulator,” IEEE Photonic Technology Letters, Vol. 10, No. 6,June 1998, pp. 805-806.

Mach-Zehnder traveling-wave electro-optic modulators with waveguidesformed in a z-cut lithium niobate substrate are disclosed in W. K.Burns, M. M. Howerton, R. P. Moeller, R. Krahenbuhl, R. W. McElhanon,and A. S. Greenblatt, “Low-Drive Voltage, Broad-Band LiNbO₃ Modulatorswith and Without Etched Ridges,” Journal of Lightwave Technology, Vol.17, No. 12, December 1999, pp. 2551-2555 and in M. M. Howerton, R. P.Moeller, A. S. Greenblatt, and R. Krahenbuhl, “Fully Packaged,Broad-band LiNbO₃ Modulator with Low Drive Voltage”, IEEE PhotonicsTechnology Letters, Vol. 12, No. 7, July 2000, pp. 792-794. A 40 Gb/sMach-Zehnder modulator with traveling wave electrode is disclosed in M.Sugiyama, M. Doi, S. Taniguchi, T. Nakazawa, and H. Onaka, “Driver-less40 Gb/s LiNbO₃ Modulator with Sub-1 V Drive Voltage”, OFC 2002,FB6-2-FB6-4.

Integrated optical photonic RF phase shifters are disclosed in E. Voges,K. Kuckelhaus, and B. Hosselbarth, “True time delay integrated opticalRF phase shifters in lithium niobate”, Electronics Letters, Vol. 33, No.23, 1997, pp. 1950-1951.

Waveguide horns for use in electro-optic modulators are disclosed inU.S. Pat. No. 6,356,673 to Burns et al. Electrodes suitable for use inelectro-optic modulators are disclosed in U.S. Pat. No. 6,381,379 toBurns et al. Additional electro-optic modulators are disclosed in U.S.Pat. No. 6,393,166 to Burns, U.S. Pat. No. 6,526,186 to Burns, and U.S.Pat. No. 6,535,320 to Burns.

Lithium-tantalate based electro-optic modulators are discussed in W. K.Burns, M. M. Howerton, and R. P. Moeller, “Performance and Modeling ofProton Exchanged LiTaO3 Branching Modulators”, Journal of LightwaveTechnology, Vol. 10, No. 10, October 1992, pp. 1403-1408.

Multiple-pass reflective electro-optic modulators are disclosed incommonly assigned patent application Ser. No. 10/165,940, now issued asU.S. Pat. No. 6,862,387, incorporated by reference in its entirety, andin M. M. Howerton, R. P. Moeller, and J. H. Cole, “Subvolt BroadbandLithium Niobate Modulators” 2002 NRL Review, pp 177-178. The low-losscompact turns increase the active length of a modulator and achieve areduction in drive voltage Vπ without sacrificing a great deal of spaceon the substrate material.

Electrodes for use in lithium niobate modulators are also discussed inR. Krahenbuhl and M. M. Howerton, “Investigations on Short-Path-LengthHigh-Speed Optical Modulators in LiNbO₃ with Resonant-Type Electrodes”,Journal of Lightwave Technology, Vol. 19, No. 9, September 2001, pp.1287-1297.

Mach Zehnder interferometers with etched ridges between the electrodesand waveguides are disclosed in W. K. Burns, M. M. Howerton, R. P.Moeller, R. W. McElhanon, and A. S. Greenblatt, “Low Drive Voltage, 40GHz LiNbO₃ Modulators”, OFC '99, pp 284-286.

BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION

An embodiment of the invention is directed to an electro-optic modulatorhaving a substrate having at least one optical waveguide formed on aface of the substrate and at least one active electrode formed on thefirst face of the substrate and aligned over the optical waveguide. Theactive electrode operates to induce a refractive index change in theoptical waveguide. The active electrode has a lower surface arrangedfacing the substrate, an upper surface arranged away from the substrate,and substantially planar side walls extending between the lower andupper surfaces. The active electrode has rounded corners between theside walls and the lower surface of the active electrode.

Another embodiment of the invention is directed to an electro-opticmodulator having a substrate having at least one optical waveguideformed on a face of the substrate, and at least one active electrodeformed on the first face of the substrate aligned over the opticalwaveguide and operable to induce a refractive index change in theoptical waveguide. The active electrode has a wider portion and at leasttwo narrower portions, with the narrower portions arranged between thewider portion and the optical waveguide. The narrower portions have asurface arranged facing the substrate, the wider portion having an uppersurface arranged away from the substrate, and substantially planar sidewalls extending between the lower and upper surfaces of the electrode.The active electrode has rounded corners between the side walls and thelower surface of the active electrode.

A more complete appreciation of the invention will be readily obtainedby reference to the following example embodiments and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an electro-optic modulator according to a firstembodiment of the invention.

FIG. 2 is a cross-sectional view of the modulator of FIG. 1 at line A-A.

FIG. 3 is a cross sectional view of an electro-optic modulator accordingto a second embodiment of the invention.

FIG. 4 is a cross-sectional view of the modulator of FIG. 3 in an activeregion at line B-B.

FIG. 5 illustrates a view of the electrodes in the bend regioncorresponding to FIGS. 3 and 4.

FIG. 6 is a graph illustrating projected drive voltage versus frequencyfor bridge electrodes according to embodiments of the invention.

FIG. 7 is of electrode transmission versus frequency for coplanarwaveguides and for bridge electrodes in accordance with an embodiment ofthe invention.

FIG. 8 illustrates the electrode loss versus electrode gap for coplanarwaveguide structures.

FIG. 9 illustrates the projected drive voltage versus frequency forcoplanar waveguide structures.

FIG. 10 illustrates the loss coefficient, dc drive voltage Vπ (DC) and20 GHz drive voltage Vπ (20 GHz) for coplanar waveguide structures.

FIG. 11 is an expanded view of the bridge electrode according to FIG. 2or 4.

FIG. 12 illustrates a lower level of a bridge electrode according to anembodiment of the invention.

FIG. 13A-B are cross sectional views of an electro-optic phase modulatoraccording to an embodiment of the invention.

FIG. 14 is a cross sectional view of an electrode in accordance withanother embodiment of the invention.

FIG. 15 is a cross sectional view of an electrode in accordance withanother embodiment of the invention.

FIG. 16A-16M illustrate steps in an exemplary method for forming anelectrode in accordance with an embodiment of the invention.

FIG. 17 illustrates another electro-optic modulator in according with anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

When configured as MZMs, the electro-optic modulators and electrodestructures described herein are useful for amplitude modulation ofoptical signals. The electrode structures described herein are alsosuitable for electro-optic phase modulators.

A waveguide is formed of a substrate material and a conveying medium.The substrate can be made of any suitable material, includingferroelectric materials such as lithium niobate, suitable for titaniumdiffused or proton exchange waveguides; lithium tantalate (LiTaO₃),which is typically used with proton exchange waveguides; barium titanate(BaTiO₃); strontium barium niobate (SrBaNbO₃); various polymers; andsemiconductor materials such as indium gallium arsenide phosphide(InGaAsP), indium phosphide (InP), gallium arsenide (GaAs), and galliumaluminum arsenide (GaAlAs). The conveying medium can be any suitablematerial which has a higher refractive index than the substrate afterformation of the optical waveguide. Since lithium niobate has good longterm stability, low optical loss, a strong electro-optic coefficient,and the ability to operate at high frequencies, the description thatfollows will be made in the context of titanium-diffused waveguides inlithium niobate substrates, although it should be understood that othermaterials can be used.

An embodiment of the invention herein provides an electro-optic device,and specifically, a Mach-Zehnder modulator with an extremely low-losselectrode. The low-loss electrodes described herein are desired for highfrequency applications and allow the incorporation of recently developedlow-loss, integrated compact turns described in U.S. Pat. No. 6,862,387,suitable for interconnecting multiple devices or regions on the samechip, increasing the device active length, and increasing the devicedensity. Compact turns in an integrated optic modulator allows anincrease in the active length of the modulator without sacrificing agreat deal of space on the substrate material. The drive voltage at dcis inversely proportional to the device length. Accordingly, compactturns also provide a means for achieving a desirable reduction in drivevoltage.

Several embodiments of low-loss electrodes are disclosed in U.S. Pat.No. 7,224,869 and U.S. Patent Publication No. US 2007-0165977 A1, andprovisional application No. 60/556,012, filed on Mar. 12, 2004. Theentire disclosures of these documents are incorporated by referenceherein. The “bridge” configuration electrode structures disclosed inthese documents are useful in electro-optic devices with long activelengths, since an increase in active length increases the overallelectrode loss.

FIG. 1 illustrates a single pass electro-optic Mach-Zehnder modulator100 in accordance with an exemplary embodiment of the invention. Themodulator includes optical waveguides formed in a substrate andelectrodes arranged to induce index changes in the optical waveguides. Asingle hot input electrode receives a RF signal from a coplanarwaveguide input horn. The base layer of the hot input electrode is splitinto two electrodes that are approximately the same width as the twooptical waveguide arms of the modulator. A wider upper portion of thehot electrode electrically connects the lower electrode portions. The RFinput signal in the electrode portions overlying the optical waveguidearms induces index changes in the optical waveguides arms. The electrodeis designed such that the velocity of the RF signal traveling down theelectrodes matches, as closely as possible, the velocity of the opticalwave traveling within the waveguide under the electrode. One of theoptical waveguide arms is reverse poled relative to the other waveguidearm. Thus, the in-phase RF input signal traveling through the electrodeinduces opposite phase shifts in the optical waveguide arms. Furtherdetails are provided in the following paragraphs. Note that FIG. 1-5 arenot to scale.

In the exemplary embodiment shown in FIGS. 1 and 2, the substrate 160 isz-cut lithium niobate, and the optical waveguide channels 140 and 142are formed by high temperature indiffusion of a titanium that has beenphotolithographically defined.

The optical waveguides are formed in a pattern such that an inputwaveguide 132 is split at a y-branch 136 into two parallel waveguides140 and 142, which are then recombined into a single output waveguide134 by an output y-branch 138. In an alternative embodiment (not shown),each of the parallel waveguides 140 and 142 can be coherently combinedusing a directional coupler providing two separate outputs from thedevice, without the output y-branch 138 and output waveguide 130.

As seen in FIG. 2, a buffer layer 170 can be arranged between the z-cutlithium niobate substrate 160 and the electrodes 110, 120, and 150 atleast in the areas that will underlie the electrodes. The buffer layer170 can be silicon dioxide or other suitable material. If the substrateis x-cut lithium niobate, no buffer layer is necessary provided that theelectrodes are beside the waveguides rather than on top of them. Inanother embodiment (not shown), the buffer layer 170 can also bedisposed over the substrate 160 in the regions between the electrodes.

As illustrated in FIGS. 1 and 2, the ground electrodes 110 and 120 arearranged on either side of and spaced apart from the central activeelectrode 150. Details of suitable materials and suitable techniques fordiffusing titanium into the substrate are found in commonly assignedU.S. Pat. No. 6,862,387.

A cross-sectional view of the active electrode is shown in FIG. 2,corresponding to the central linear active portion of the modulatorbetween the turns. The active or hot electrode 150 is arranged over bothof the parallel waveguides 140 and 142. The hot electrode 150 has abridge configuration, with two lower portions 152 and 154 alignedpreferably directly over the parallel waveguides 140 and 142. The hotelectrode's lower portions 152 and 154 are approximately the same widthas the parallel waveguides 140 and 142. The lower portions of theelectrode 150 are separated by a distance that is approximately the samewidth as the distance between the optical waveguide channels 140 and142. The space 162 can be empty (e.g., filled with air) or can be filledwith an electrically insulating material preferably with a lowdielectric constant.

The upper portion 156 of the bridge-shaped hot electrode 150 has a widthapproximately the same as the optical waveguides 140 and 142 plus thedistance between the optical waveguides, and is aligned over and inphysical contact with the lower portions of the electrode.

FIG. 1 shows the electrode 150 with the upper portion 156 cut away toillustrate the alignment of the electrode legs or lower portions 152 and154 over the optical waveguide arms 140 and 142. In the region 144 wherethe optical waveguide arms are approximately linear and parallel, thelower parts of the electrode 150 are aligned over the optical waveguidearms.

The active electrode and ground electrodes are configured to match asclosely as possible the velocity of the RF signal in the activeelectrode 150 and the velocity of the optical signal in the opticalwaveguide channels 140 and 142. For example, the overall width of theactive electrode is not limited to the width of the optical waveguide,as in some conventional MZMs, but may be equal to the sum of the widthsof the optical waveguides plus the edge-to-edge separation distancebetween the two optical waveguides. The larger size of the bridgeelectrode allows much wider gap spacing between the active centerelectrode 150 and the ground electrodes 110 and 120 than in conventionalMZM designs. The wider gap spacing allows the electrodes to be made muchthicker (in the z direction of FIG. 2), resulting in considerably lowerelectrical losses in the electrodes.

At either end of the active region 144, the two electrode legs 152, 154that form the base of the electrode bridge 150 are recombined andpreferably terminated with a resistor equal to the characteristicimpedance of the transmission line. For an unamplified input signal,this configuration approximately provides an additional 2^(1/2) directimprovement over an electro-optic modulator with one hot electrode overone waveguide and a ground plane over the waveguide and an equivalentactive modulation length.

In the exemplary embodiment of FIG. 1-2, an optical source such as alaser (not shown) provides optical energy into the optical waveguideinput 132. The device of FIG. 1-2 is configured to receive opticalenergy at the 1.5 micron wavelength, however, optical energy at otherwavelengths is also suitable. For example, optical energy at the 1.3micron wavelength can be carried by the optical waveguide 132. Thedimensions of the optical waveguide can also be adjusted to carryoptical energy at longer or shorter wavelengths.

Another exemplary embodiment of an electro-optic modulator isillustrated in FIG. 3-5.

FIG. 3 illustrates a multiple-pass electro-optic Mach-Zehnder modulator300 in accordance with another exemplary embodiment of the invention.The modulator 300 includes optical waveguides formed in a substrate andelectrodes arranged to induce index changes in the optical waveguides. Asingle hot input electrode 350 receives a RF signal from a coplanarwaveguide input horn.

The base layer of the hot input electrode 350 is split into twoelectrodes that pass over two optical waveguide arms of the modulator.The RF input signal induces index changes in the optical waveguidesmodulating the velocity of the propagating optical signals. One of theoptical waveguide arms is reverse poled relative to the other waveguidearm. Thus, the in-phase RF input signal traveling through the electrodesinduces opposite phase shifts in the optical waveguides. The electrodeis designed such that the velocity of the RF signal traveling down theelectrodes matches, as closely as possible, the velocity of the opticalwave traveling within the waveguide under the electrode. Further detailsare provided in the following paragraphs.

As illustrated in FIG. 3, the active region is folded back and forth inorder to increase the interaction length, resulting in a reduced drivevoltage. The modulator 300 includes a long active modulation region 380with five approximately linear portions in which the active electrodeoverlies the waveguide arms. Compact reflective waveguide turns 382 areprovided at the ends of the linear portions, as disclosed in U.S. Pat.No. 6,862,387 and discussed in the following paragraphs.

The optical waveguides are formed in a pattern such that an inputwaveguide 332 is split at a y-branch 336 into two parallel waveguidechannels 340 and 342. At the end of the optical waveguide opposite theinput portion 332, the optical waveguide channels 340 and 342 eachinclude a compact 180 degree waveguide s-bend turn. As illustrated inFIG. 3, the modulator 300 includes reflective surfaces 381 and 383 atthe edges of the substrate 360 adjacent the optical waveguide turns 382.The reflective surfaces can be gold or dielectric mirrors, or any othersuitable material with a high reflectivity at the wavelength of theoptical energy carried by the optical waveguides.

As discussed in U.S. Pat. No. 6,862,387, reflective surfaces 381 and 383can be formed on an edge of the substrate 360 which should be polishedto a smooth surface before application of the reflective surface.Smoothness of the substrate edge in the vicinity of the opticalwaveguides should be about or better than ⅕ of the typical wavelength of1.5 microns or about 0.3 microns. Waveguide portions 340 a and 340 b,for example, meet at an apex 343. As discussed in U.S. Pat. No.6,862,387, the distance of the apex of the optical waveguide to thereflecting surface 381 in the y direction can be between 0 and plus orminus 14 microns. The optical waveguides 340 and 342 can be betweenabout 50 to 500 microns apart, measured from the centerline of theoptical waveguides, and should be sufficient to limit coupling via theevanescent wave effect between waveguide arms. The reflective surface381 is not exactly at 90 degrees to the incoming light in the opticalwaveguide arms. The offset from 90 degrees can be below about 10degrees, providing a compact modulator with more than one pass andtherefore a longer active region. Reflective surfaces can also be formedby etching a groove in the substrate, and depositing a material that ishighly reflective at the optical frequency, as discussed in U.S. Pat.No. 6,862,387.

Reflective surfaces can also be formed by etching a groove in thesubstrate, and making use of total internal reflection if the incidentangle is sufficiently large.

Light in the optical waveguide arm 340 a will propagate along the lowers-bend half and will be reflected by reflective surface 381, proceedingalong the upper s-bend half to the optical waveguide arm 340 b.Similarly, light in the optical waveguide arm 342 a will propagate alongthe lower s-bend half and will be reflected by reflective surface 381,proceeding along the upper s-bend half to the optical waveguide arm 342b. After being reflected by the reflecting surfaces and modulated in theactive regions 380 between the compact s-bend turns, the signals in theoptical waveguide arms will be recombined at the output y-branch 338 andwill exit the modulator through output waveguide 334. In an alternativeembodiment (not shown), each of the parallel waveguides 340 and 342 canbe coherently combined using a directional coupler providing twoseparate outputs from the device, without the output y-branch 338 andoutput waveguide 334.

In the active regions 380 of the modulator, the optical waveguide arms340 and 342 follow the path of the electrodes 310, 320, and 350. Compactreflective waveguide turns in the bending regions allow compact spacingof the adjacent active regions in the x-direction. In alternativeembodiment (not shown), the optical waveguides can follow a semicircularpath in the bending regions, however, this would require several timesmore space to contain the same number of active regions or transition toa high index gradient waveguide.

The substrate 360 is preferably formed of a crystalline material havinga high electro-optic coefficient such as lithium niobate or lithiumtantalate. Other suitable materials include barium titanate, strontiumbarium niobate, a polymer, indium gallium arsenide phosphide, indiumphosphide, gallium arsenide, and gallium aluminum arsenide. In theexemplary embodiment shown in FIGS. 3 and 4, the substrate 360 is z-cutlithium niobate, and the optical waveguide channels 340 and 342 areformed by high temperature indiffusion of a titanium that has beenphotolithographically defined.

As illustrated in FIGS. 3 and 4, the ground electrodes 310 and 320 arearranged on either side of and spaced apart from the centrally arrangedhot or active electrode 350. A cross-sectional view of the activeelectrode 350 is shown in FIG. 4, corresponding to the central linearactive portion of the modulator between the turns. The active or hotelectrode 350 is arranged over both of the parallel waveguides 340 and342. The hot electrode 350 has a bridge configuration, with two lowerportions 352 and 354 aligned preferably directly over the parallelwaveguides 340 and 342 in the linear portions of the active region. Asillustrated in FIG. 4, the hot electrode's lower portions 352 and 354are approximately the same width as the parallel waveguides 340 and 342.The lower portions of the electrode 350 are separated by a distance thatis approximately the same width as the distance between the opticalwaveguide channels 340 and 342. The space 362 can be empty (e.g., filledwith air) or can be filled with an electrically insulating materialpreferably with a low electro-optic coefficient.

As illustrated in FIG. 4, the upper portion 356 of the bridge-shaped hotelectrode 350 has a width approximately the same as the opticalwaveguides 340 and 342 plus the distance between the optical waveguides,and is aligned over and in physical contact with the legs or lowerportions of the electrode. The electrode can be formed in two or moresteps, with a base layer being deposited first to form the lowerelectrode portions, and the upper portion of the electrode being formedof subsequently applied layer or layers of metallization. The upperportion 356 of the hot electrode 350 can have a width approximatelyequal to the widths of the electrode legs plus the gap between the legs,as seen in FIG. 4. The upper portion must be in electrical contact withthe electrode base portions, although it is not necessary that the upperportion of the electrode be precisely aligned over the electrode baseportions. The width of the upper portion can be slightly less than thewidths of the electrode legs plus the gap between the legs. The width ofthe upper portion can also be greater than the widths of the electrodelegs plus the gap between the legs, as seen in the electrode embodimentsillustrated in FIG. 13A-B.

FIG. 3 shows the electrode 350 with the upper portion 356 cut away toillustrate the alignment of the electrode legs or lower portions 352 and354 extending over and aligned with the optical waveguide arms 340 and342 in the parts of the active region 380 where the optical waveguidesare approximately linear and parallel.

A buffer layer 370 can be arranged between the z-cut lithium niobatesubstrate 360 and the electrodes 310, 320, and 350 at least in the areasthat will underlie the electrodes. The buffer layer 370 can be silicondioxide or other suitable material. If the substrate is x-cut lithiumniobate, no buffer layer is necessary. The buffer layer 370 can also bedisposed over the substrate 360 in the regions between the electrodes.

The electrode structure may be terminated with a resistor with thecharacteristic impedance of the transmission line.

The active electrode and ground electrodes are configured to match asclosely as possible the velocity of the RF signal in the activeelectrode 350 and the velocity of the optical signal in the opticalwaveguide channels 340 and 342 in the linear region between the turns.For example, the overall width of the active electrode is not limited tothe width of the optical waveguide, as in some conventional MZMs, butcan be equal to about twice the width of the a waveguide plus theedge-to-edge separation between the two waveguides, or greater. Thelarger size of the bridge electrode allows much wider gap spacingbetween the active center electrode 350 and the ground electrodes 310and 320. The wider gap spacing allows the electrodes to be made muchthicker (in the z direction), resulting in lower electrical losses inthe electrodes.

The surface of the substrate 360 can be removed by etching or othersuitable removal technique in the gaps between each ground electrode310, 320 and the hot electrode 350. The substrate surface can also beremoved in the space 362 between the two lower portions of the activeelectrode 350. FIG. 4 illustrates that in the linear part of the activeregion of the modulator, the upper surface of the lithium niobatesubstrate 360 has been removed by etching in the space 362 between theactive electrode legs 352, 354 and in the region 364, 366 between theground electrodes 310, 320 and the active electrode 350. Removing aportion of the substrate appears to improve the velocity matchingbetween the RF and optical signals by increasing the velocity of the RFsignal in the active electrode. Removal of a portion of the substratealso can affect the impedance of the electrode. The bridge electrode,when optimized to match the RF and optical velocities, has acharacteristic impedance lower than 50 ohms without etching.

Etching the substrate surface in the bend regions is difficult toaccomplish without damaging the optical waveguide crossings near thereflective s-bends. To impedance match the bridge electrode in theturning region, the height of the electrode is reduced to a height lessthan the height in the linear active region to obtain 50 ohms withoutetching the lithium niobate substrate. For example, when the bridgeelectrode height is 90 microns in the linear part of the modulator, thebridge electrode height in the turning region is 30 microns thick. Thereduced height of the electrodes in the bending region, however, cancause a mismatch between the velocities of the RF and optical signals.Therefore, the physical lengths of the optical waveguides and theelectrodes are selected to match the total transit time of the opticaland electrical signals through the bend region, ensuring they are inphase as they transition through the turn into the next linear activeregion. A three dimensional electromagnetic model can be used todetermine the appropriate lengths of the optical waveguides andelectrodes in the bend region and linear active regions. FIG. 5illustrates the bridge electrode 350 and the ground electrodes 310 and320 in the turning region of the modulator.

The side walls of the electrodes can be perpendicular to the substrate,or can be slightly flaring outward so the upper portion of theelectrodes are wider than the lower portions, as illustrated in FIGS. 2and 4. The upper portion of the electrode side walls could also benarrower than the lower portion (not shown).

In an alternative embodiment (not shown), the optical waveguidesunderlie the hot electrode even in the bend region, and reflectives-bends are not provided at the edges of the substrate. This embodiment,while having the advantage that etching can be performed between theelectrodes without harming the optical waveguides, does not includecompact turns, so requires more space on the substrate.

Modulators based on x-cut lithium niobate have typically been limited toa lower frequency range than modulators based on z-cut lithium niobatebecause for x-cut modulators the optical waveguides must go between theelectrodes to utilize the larger electro-optic coefficient, r33. Thislimits the electrode gap width. Narrow gap widths require a shorterelectrode height for velocity matching which in turn results in higherlosses than experienced for the tall electrodes allowed for z-cutdevices. As a result for x-cut devices there is a trade-off betweenhigher drive voltage Vπ and frequency response. However, forapplications in which the frequency range and drive voltage are notcritical, the bridge electrode described herein can be included in x-cutlithium niobate based modulators. For such modulators, the lowerportions of the bridge electrode can be arranged on either side of awaveguide, rather than directly aligned over the optical waveguide arms.

The following is a description of a suitable method for forming theelectro-optic modulators of FIG. 1-5. A substrate is suitably selectedas a z-cut optical-grade commercial lithium niobate wafer. The z planeis the plane perpendicular to the crystal axis (z) and is the largestface of the substrate. The wafer can be approximately 3-4 inches indiameter with a 1 mm thickness, although larger or smaller size waferscan be used. The wafer is cleaned in trichloroethylene, acetone,methanol, detergent, and deionized water. Titanium is sputtered at roomtemperature over the z face to a thickness of 600 angstroms. Opticalwaveguides can be formed in the substrate photolithographically byspin-coating photoresist on the substrate, prebaking the photoresist at90 degrees C. for 25 minutes, exposing the photoresist to UV lightthrough the optical waveguide photomask, with the optical waveguidesaligned along the y-axis of the substrate. The photoresist is thendeveloped to eliminate it, and postbaked at 110 degrees C. for 45minutes to fully harden it in the optical waveguide regions. Finally,the titanium is etched away by the use of ethylene diamine tetraaceticacid (EDTA). The final titanium strip width after etching is 8 um andproduces single-mode waveguides after indiffusion of titanium. Thesubstrate is then placed in a furnace and heated to an elevatedtemperature of 1000 degrees C. for 10 hours in wet oxygen. Thistechnique produces high quality optical waveguides with very lowpropagation losses in straight channels, for example, losses ofapproximately 0.1 dB/cm.

A poling mask is used to define a poling electrode over one of the twowaveguide regions. A voltage is applied to the poling electrode toreverse the ferroelectric domains in only one of the two waveguides. Thenet effect of this poling is a reduction in modulator drive voltage.After reverse poling is completed, the poling electrode is chemicallyetched away. An etch mask is then used to define the areas on thesubstrate that are to be etched by ion milling or another suitabletechnique. The lithium niobate ridge (unetched area) can be slightlywider than the electrode footprint, for example, to minimize opticalloss in the active region associated with roughness of the etchedsurface.

A buffer layer of silicon dioxide is deposited over the wafer afteretching of the substrate. Alternatively, the buffer layer can bedeposited prior to etching.

The layering process for the electrodes is illustrated by FIG. 4. Afirst modulator electrode mask is used to define the foundation (lowerportion or legs) of the bridge electrode, and a first layer of theground planes. The electrodes are plated everywhere to a height h1 of 20microns. In a preferred embodiment, the electrodes are formed of gold.For structural support, the volume between the legs of the bridgeelectrode can be filled with a polymer or other suitable material beforethe upper layer is applied.

A second modulator mask is used to define the center hot electrode andall other electrodes, and the electrodes are plated to an additionalheight h2 of 10 microns. This completes the formation of the 30 micronelectrode height in the turning region.

A third modulator mask is used to form the mold for the linear part ofthe modulator between the turns, where the hot electrode and the groundplanes are plated with another 60 microns of electrode material (h3),for a total of 90 microns in height (in the z direction).

The input and output horns couple RF input energy to the first activeregion and from the last active region of the modulator. The input andoutput horns can have a thickness of approximately 20 microns. In thisembodiment, the horns are not impedance matched. Their length is lessthan the wavelength of the RF energy, so impedance matching is notnecessary. The impedance, effective refractive index, and gold thicknessin microns of the FIG. 3-5 embodiment is as follows: Z gold region(ohms) n eff thickness characteristics linear active 48 2.14 90 micronsvelocity and impedance region matched turns 48 3.41 30 microns impedancematched horns 38 2.6 20 microns not impedance matched (RF to active)

A significant advantage of the modulators described herein is that theyare inherently chirp-free. For a Mach-Zehnder modulator, chirp is theratio of the phase modulation to the amplitude modulation where thephase modulation is the time averaged phase modulation for bothwaveguides in the Mach-Zehnder modulator. Previous Mach-Zehndermodulators that apply different electrical fields to the two waveguidearms are susceptible to chirp due to the differential in the electricfield applied to each electrode over the two waveguides. In MZMs withone electrode at ground potential (zero electrical field) and the otherelectrode at the maximum applied voltage, the optical waveguide underthe grounded electrode has only a small contribution to the averagephase while the other optical waveguide has most of the modulation phaseshift.

In contrast, the electrode bridges configurations disclosed herein andillustrated in FIG. 1-5 allows the same electrical field to be appliedto both waveguides. Therefore, the magnitude of the optical signal'sphase shift is the same in each waveguide. Since one of the opticalwaveguides is reverse poled, the sign of the phase modulation in one ofthe optical waveguides is reversed relative to the other waveguide. Thisresults in a time averaged phase modulation of zero for equal lengthwaveguides. Thus, the bridge structure electrodes are inherentlychirp-free.

Another advantage of the electrode configuration disclosed herein isthat the fabrication tolerances for the bridge structure are reducedcompared to coplanar waveguide structures. Although more masks are usedto define the bridge structure, only the bottom electrode layer of 20 μmrequires precise alignment during the photolithography stage. Incontrast, other low-loss coplanar waveguides not having a bridgestructure require thicker electrodes in a single step (e.g., greaterthan 40 microns), making the alignment during the photolithographyprocess much more difficult.

For a number of modulator applications a modulator drive voltage of 0.5V or less is desirable. Further, operation without a low noise amplifierbetween the electrical source (such as an antenna) and the modulator canbe desirable due to a lack of electrical power locally, to minimizelocal power consumption, or to eliminate distortion products created bythe nonlinearities of the amplifier. For example, with drive voltages of0.5 V or less, microwave transmission from antennas can be accomplishedwithout any amplifier at the antenna and with RF gain in the fiber opticlink and noise figure of the same order as a low noise amplifier.

FIG. 6 and the following table illustrate the projected drive voltagefor single pass and multi-pass modulators using the bridge electrodeconfiguration described herein. The modulator illustrated in FIG. 3-5having a low-loss bridge electrode structure, five active regions, andcompact waveguide turns provides a Lithium niobate Mach-Zehndermodulator design with a projected drive voltage of 0.6 V or less through20 GHz. The modulator illustrated in FIGS. 1-2 and 3-5 are projected tohave a drive voltage of 1.4 V or less through 20 GHz.

Model at 1.55 micron optical signal wavelenght: Vπ (Volts) InteractionPasses DC 10 GHz 20 GHz Length (cm) 1 1.2 1.33 1.4 7.4 3 0.4 0.55 0.6723.4 5 0.2 0.4 0.57 39.4

Achieving the low drive voltage over the 0-20 GHz frequency range isvery useful for applications where amplifiers cannot be used between theRF source and the modulator, particularly where weight, size, powerdissipation and power consumption are issues. Further, by eliminatingamplifiers in RF systems, the lack of amplifier noise and distortiongenerated by the inter-modulation products of the amplifier improves thesensitivity of the RF system. System applications include wing mountedantenna array telemetry, space based systems and commercial and militarytelecommunication systems in which significant cost savings can beachieved while increasing reliability. With drive voltages of less than0.5 V, microwave signals from antennas can be received without anyamplifier at the antenna and transmitted over optical fiber with RF gainin the link and noise figure of the same order as a low noise amplifier.

The modulators and electrodes of FIG. 1-5 are also suitable formodulation at frequencies greater than 20 GHz.

The following discussion is provided to clarify the advantages ofproviding a single electrode with reverse poled waveguides compared toother modulator types. Four possible configurations of a single passmodulator are as follows: (1) a hot electrode over one waveguide with noelectrode over the other waveguide; (2) a hot electrode over onewaveguide with a ground plane over the other waveguide; (3) a hotelectrode over one waveguide with a second hot electrode over the otherwaveguide, operating with two electrical driving signals which are 180degrees out of phase; and (4) a hot electrode over one waveguide with asecond hot electrode over the other waveguide operating with either asingle electrical signal or two electrical driving signals in phase,with the optical waveguide under one electrode being reverse poled tochange the sign of the modulation.

Push-pull is a method of combining two signals that are out of phase toget more modulation effect between the two waveguides forming theMach-Zehnder interferometer. The push pull method can be implementedthrough electrode or optical design. The electrode configurations of (3)and (4) are referred to as push-pull. Electrode designs consistent withconfiguration (2) can also provide a small increase in modulationefficiency compared with configuration (1) due to non-negligible fieldintensity under the ground electrode. It should also be noted that thepush-pull configurations (3) and (4) are sometimes erroneouslyconsidered to provide twice the modulation due to the push-pullconfiguration. In the case of a RF source without amplification, thepower must be divided between the two electrodes. Since these modulatorsrespond to the voltage developed across the electrodes, the maximumimprovement over configuration (1) modulators is 2^(1/2) for typical 50ohm systems rather than a full factor of 2.

For applications where there is no amplifier between the RF drivingsource and the modulator, configurations (2) and (4) are the mostpromising. Configuration (1) has the poorest effective response of allconfigurations and configuration (3) requires a wideband power dividerand a low-loss 180 degree RF phase shifter.

In order to maximize the modulator bandwidth and response, travelingwave modulator designs are employed or device length is shortened at theexpense of increased drive voltage requirements. The modulators aredesigned such that the velocity of propagation of the optical wave ismatched to the velocity of the microwave by adjusting the geometry ofthe electrode. For high frequency traveling wave modulators, as theactive region length is increased to reduce the dc drive voltage, theimpact of increased electrode losses becomes more significant, causingthe response to deteriorate rapidly at high frequency. Therefore, inorder to take advantage of increased active region length enabled bycompact reflective turns, extremely low loss electrode structures aredesired. The electrode designs described in this disclosure can providelosses which are lower than conventional structures, ultimately enablingless than 0.5 V drive voltages.

FIG. 7, a graph of electrode transmission versus frequency, illustratesthat the bridge electrodes as illustrated in FIGS. 2 and 4 providesubstantially reduced loss over other coplanar waveguide designs. CurvesA show measured results for a single pass modulator with a 4.5 cminteraction length coplanar waveguide electrodes, a 25 micron gap, and a32 micron height. Further details are provided in M. M. Howerton, R. P.Moeller, A. S. Greenblatt, and R. Krahenbuhl, “Fully packaged Broad-bandLiNbO3 Modulator with Low Drive Voltage,”, IEEE Photon. Technol. Lett.12, 792-794 (2000), and fitted loss coefficient of 0.043 (GHz^(1/2)-cm)⁻¹ for this modulator. Curves B represent a modulator withimproved coplanar waveguide electrode morphology, resulting in a losscoefficient of 0.025 (GHz ^(1/2)-cm)⁻¹. Curves C represent measured andcurve fitted results for a modulator with a coplanar waveguide gap widthof 45 microns and electrode height of approximately 45 microns. The losscoefficient for this modulator is α₀=0.015 (GHz ^(1/2)-cm)⁻¹. The heightfor this electrode was slightly short of that appropriate for velocitymatching for the curve C modulator.

Curves D of FIG. 7 shows the electrical transmission for the bridgestructure disclosed herein with a 90 micron electrode height in theactive region, without etching between the lower portions of the activeelectrode, and without etching between the active electrode and theground planes. The fitted curve D corresponds to a loss coefficient of0.008 (GHz ^(1/2)-cm)⁻¹.

Further reductions in electrode losses of CPW structures are possible byincreasing the electrode gap between the center electrode and the groundplane of coplanar waveguide electrodes and increasing the thickness ofthe electrode. Curve E represents the projected loss of an optimizedbridge design with etching, calculated using a three dimensional finiteelement model. The projected loss coefficient is 0.0065 (GHz^(1/2)-cm)⁻¹, which includes both resistive and dielectriccontributions.

Thus, the bridge electrode design disclosed herein provides asubstantial reduction in electrode loss compared to other coplanarwaveguide designs shown in FIG. 7.

FIG. 8 illustrates the electrode loss versus electrode gap for coplanarwaveguide structures. As the gap is increased, the electrode heightshould also be increased to ensure that the optical velocity and RFvelocity are matched. FIG. 9 illustrates the electrode transmissionversus frequency for coplanar waveguide structures. FIG. 10 illustratesthe loss coefficient, dc drive voltage Vπ (DC) and 20 GHz drive voltageVπ (20 GHz). As illustrated in FIGS. 9 and 10, drive voltages of lessthan 0.5 V at 20 GHz are not feasible for the configuration (2) coplanarwaveguide.

As the modulators illustrated in FIG. 1-4 each have reverse-poledwaveguide arms and a single hot electrode, they correspond best to theconfiguration (4) structure discussed in previous paragraphs. Thus, theFIG. 1-4 modulators provide significant advantages over coplanarwaveguide structures.

FIG. 11 is an expanded view of the substrate and bridge structure ofFIG. 2 or 4. FIG. 12 illustrates the first layer of metallizationforming the base level of an active electrode 150 or 350 shown in FIG.1-2 or 3-5. The active electrode 150 is spaced apart from each groundplane by 100 microns, and each of the electrode legs is 8 microns inwidth. The two optical waveguides should be separated enough to limitoptical crosstalk. Deep ion etching as illustrated in FIG. 11 can limitcrosstalk between the optical waveguides while minimizing the gapbetween the optical waveguides.

The upper portion of the active bridge electrode can extend in the widthdirection beyond the outer edges of the electrode base layer. A largesurface area is preferred for decreasing the loss in the electrode.

FIG. 13A-B illustrate cross sectional views of an electro-optic phasemodulator according to another embodiment of the invention. An opticalwaveguide 640 is formed in a substrate 660 by the titanium indiffusionmethod described above, or by another suitable method. An electrode 650and ground plane 610 are formed on the substrate. The hot electrode isspaced apart from a ground plane 610. As seen in FIG. 13A, the electrode650 has a lower portion 652 aligned with the optical waveguide 630 in amanner such that applying a RF signal to the electrode induces a changein the refractive index of the optical waveguide, in turn producing avelocity matched optical signal in the optical waveguide. The lowerportion 652 preferably has a width about the same as the width of theoptical waveguide. The upper portion 656 of the electrode is wider thanthe lower portion, and preferably is at least twice, and morepreferably, at least three times as wide as the lower portion. Forexample, a presently preferred width for the upper portion of theelectrode is 32 microns when the lower portion is 8 microns wide, withthe electrode having a total height of 90 microns. A polymer or othersuitable electrically insulative material can be deposited on thesubstrate in the areas 612 and 614 on either side of the lower portionof the electrode to provide a stable platform on which to deposit thesubsequent layer or layers of the electrode.

As seen in FIG. 13B, the electrode can also have more than one lowerportion to provide a stable base for the subsequently deposited layersof the electrode. In this embodiment, only one of the lower portions 652will be aligned over the waveguide.

FIG. 14 is an expanded view of a bridge electrode 700. In thisembodiment, the bridge electrode 700 has lower portions 710 and 720arranged over parallel optical waveguides 730 and 740. A buffer layer ispositioned between the electrode 700 and the Z-cut lithium niobatesubstrate 760.

The parallel optical waveguides 710 and 720 can be formed by titaniumdiffusion into the substrate. A groove can be etched in the substratebetween the optical waveguides, with the groove being about the samewidth as the width of the waveguides 730 and 740. Grooves are alsoetched in the substrate between each waveguide and the ground planes 770and 780. Etching grooves in the substrate around the waveguide can beadvantageous for two advantages. First, the etching concentrates theinduced field in the optical waveguide. The high dielectric constant ofthe waveguide material restricts the field from spreading out into theair in the surrounding etched region on either side of the waveguides.

In this embodiment, the unetched lithium niobate substrates in thewaveguide regions are about 12 microns in width, and the electrode legsare about 8 microns in width. The width of the electrode upper portionis about 32 microns, and the overall electrode height is about 90microns. The lower portions of the electrode have a width of about 8microns.

The electrode 700 has an upper surface 750 that is the farthest pointfrom the substrate top surface of the electrode 760. The corners 752 and754 between the upper surface 750 and the side walls are sharp.

The electrode 700 can be formed by a process of gold plating the lowerportions of the electrode and the upper portion of the electrode, in themanner discussed in preceding paragraphs addressing FIG. 4, or by anyother suitable means.

FIG. 15 is a cross sectional view of a bridge electrode 800 inaccordance with another embodiment of the invention. The substratestructure and waveguides are identical to those in FIG. 14. The topsurface of the electrode 800 has rounded corners 811 and 812. Eachcorner 811, 812 has a radius of curvature of about ¼ to about ½ of thewidth of the electrode. In the embodiment shown in FIG. 15, the corners811, 812 have a radius of curvature of 16 microns. The electrode widthis about 38 microns, resulting in a flat upper surface 810 of about 6microns.

The parallel lower portions 860 and 870 of the electrode each haverounded corners 862, 864 and 872, 874.

The contact area of the electrode legs should have a width that is aboutequal to the width of the optical waveguide. In this example, therounded corners 862, 864, 872, 874 have a radius of curvature of aboutfour microns, the width of the optical waveguide is about 8 microns, andthe corresponding contact areas of the electrode legs are about 8microns in width. The rounded corners of the electrode legs can have aradius of curvature of about half of the contact area width, although itcan be greater or smaller. In this embodiment, the radius of curvatureis about 4 microns, resulting in a width of each of the lower portionsof about 16 microns.

The interior corners 882, 884 of the electrode 800 do not concentratethe electrical field, so it is not necessary to radius these corners toreduce loss. However, the corners 882, 884 can be curved if desired.

The curved exterior corners of the electrode are believed to result inan electrical field in the electrode that is less concentrated in thecorner regions, reducing the electrical loss. The rounded corners of thelower portions of the electrode allows the overall width of theelectrode to be wider, and the comparatively wider electrode 800 resultin lower loss, compared to the electrode 700 of FIG. 14. Thesecharacteristics allow the FIG. 15 electro-optic modulator to have anelectrical loss of about 0.2 decibels per centimeter, compared with theFIG. 14 electro-optic modulator's electrical loss of about 0.3 decibelsper centimeter. The low-loss properties of the rounded corner bridgeelectrode allows an electro-optic modulator to operate at lower drivevoltages.

The region between the electrode legs 860 and 870 and the ground planes840 and 830 are etched, to concentrate the induced field in the opticalwaveguide and restrict the field from spreading out into the air in thesurrounding etched region on either side of the waveguides.

A wider gap between the active electrode 800 and the ground electrodes830 and 840 allows the electrodes to be taller, reducing the electricalloss. A gap of about 100 microns between the active electrode 800 andthe ground electrodes 830 and 840 is appropriate for optical modulationat frequencies up to about 20 GHz. The gap can be wider for lowerfrequencies. For example, the gap can be between 150 and 200 microns forfrequencies in the six to twelve gigahertz range.

Aspects of the invention also include methods for making the roundedcorner electrodes described herein. An example of a method formanufacturing the rounded cornered bridge electrode of FIG. 15 is shownin FIG. 16A-M.

FIG. 16A illustrates the substrate 900, a buffer layer 902, a metal seedlayer 904, and etched regions 906, 907, and 908. The titanium diffusedwaveguides (not shown) have been poled. It is preferred that the groovesare etched in the substrate before poling the electrodes, in order tominimize waste due to errors in the etching process, however, it is alsopossible to pole the electrodes before etching the grooves in thewaveguides.

a) If using a positive photoresist, apply the photoresist 910 over theelectrode region, as shown in FIG. 16B.

Next, expose the desired portion of the photoresist to light, using amask 912 to prevent light from reaching the remaining portion of thephotoresist, as illustrated in FIG. 16C. Use a solution to wash away ordissolve the exposed photoresist. FIG. 16D shows the resulting mold thatremains after the exposed photoresist is removed. The mold has grooves914, 916, 918, and 920 in which the electrodes will be formed.

FIGS. 16E, 16F, and 16G are expanded views of the active bridgeelectrode region. FIG. 16E illustrates application of a photoresistsolution with sufficient viscosity to the grooves 916 and 918 to fill inthe corners of the mold. Capillary action causes the liquid to form aconcave surface in the corners 922, 924, 926, and 928. The photoresistsolution is allowed to harden.

As shown in FIG. 16F, the liquid may cover the bottom of the mold. Ifso, another mask can be used to expose a narrower section of theelectrode, and a solution can be used to wash away/dissolve the excessphotoresist in the center of the electrode base, to expose the metalseed layer 904 and form the flat base 932 and 934 of the electrode.

What remains is a mold with grooves and rounded edges in which theelectrodes will be formed.

As illustrated in FIG. 16H, a first layer of the electrodes are formedon the exposed metal seed layer in the mold by plating metallicelectrode material into the grooves between the masks. The first layerof electrode material forms the lower portions 940 and 942 of the activebridge electrode and a first layer of the ground electrodes 936 and 938.

Repeat the steps of applying a photoresist, exposing portions of thephotoresist to light, dissolving the exposed photoresist, andelectroplating the metallic electrode material as necessary to build theelectrodes to the desired height and to form the bridge structure.

For example, in FIG. 16I, a positive photoresist 944 has been appliedover the electrode region, and portions of the photoresist are exposedto light, with other portions protected by a mask 946. In FIG. 16J, theexposed photoresist has been dissolved or washed away, forming grooves948, 950, 952 in which the next layer of electrode will be formed. Metalelectrode material 954, 956, and 958 is plated into the grooves to buildthe electrodes in height, as shown in FIG. 16K.

As seen in FIG. 16L, to form the rounded corners on the upper surface ofthe active electrode, progressively narrower layers of gold material areplated onto the center electrode.

As a final step, the electrode region is exposed to light, and asolution is used to dissolve the remaining photoresist, leaving thecompleted center electrode and ground electrodes in place, asillustrated in FIG. 16M.

Note that if the photoresist is negative, the process is similar, exceptthat a mask is used to block light from reaching the areas intended tobe dissolved, rather than the areas that will not be dissolved.

Rounded corner electrodes are not limited to the bridge electrodeconfiguration of FIG. 16. FIG. 17 illustrates another embodiment of anelectro-optic modulator having rounded corners on the electrode. FIG. 17is a cross sectional view of the electrode portion of an electro-opticmodulator with rounded upper and lower corners for the active electrode970. Note that FIG. 17 is not to scale, and the distance between theactive electrode and the ground electrodes 980 and 990 is larger than itappears. A plan view of the modulator shown in FIG. 17 is similar to theFIG. 1A or FIG. 1B in U.S. Pat. No. 5,416,859, incorporated by referenceherein in its entirety.

The rounded corner electrode and electro-optic modulator of FIG. 17 aremade in a manner similar to the method described for producing thebridge electrode of FIG. 15.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that the claimed invention may be practiced otherwise than asspecifically described.

1. An electro-optic modulator comprising: a substrate having at leastone optical waveguide formed on a face of the substrate; at least oneactive electrode formed on the first face of the substrate and alignedover the optical waveguide; the active electrode operating to induce arefractive index change in the optical waveguide, the active electrodehaving a lower surface arranged facing the substrate, an upper surfacearranged away from the substrate, and substantially planar side wallsextending between the lower and upper surfaces, wherein the activeelectrode has rounded corners between the side walls and the lowersurface of the active electrode.
 2. The electro-optic modulator of claim1, wherein the radius of curvature of the rounded corners is betweenabout 25% and 75% of the width of the contact area of the electrode. 3.The electro-optic modulator of claim 1, wherein the substrate comprisesZ-cut lithium niobate.
 4. The electro-optic modulator of claim 3,wherein the substrate further comprises a silicon dioxide buffer layer.5. The electro-optic modulator of claim 1, wherein the substrate furthercomprises a metal seed layer arranged over the silicon dioxide bufferlayer.
 6. The electro-optic modulator of claim 1, wherein the radius ofcurvature of the rounded corners is between about half of the width ofthe contact area between the active electrode and the substrate.
 7. Theelectro-optic modulator of claim 1, wherein the width of the contactarea between the active electrode and the substrate is about half of themaximum width of the active electrode.
 8. The electro-optic modulator ofclaim 1, wherein the upper surface of the active electrode is convex. 9.The electro-optic modulator of claim 1, wherein the active electrode hasrounded corners between the side walls and the upper surface.
 10. Theelectro-optic modulator according to claim 9, wherein the radius ofcurvature of the upper surface corners is between about ¼ and ½ of themaximum width of the active electrode.
 11. The electro-optic modulatorof claim 1, wherein the substrate is a Z-cut lithium niobate.
 12. Anelectro-optic modulator comprising: a substrate having at least oneoptical waveguide formed on a face of the substrate; and at least oneactive electrode formed on the first face of the substrate aligned overthe optical waveguide and operable to induce a refractive index changein the optical waveguide; the active electrode having a wider portionand at least two narrower portions, the narrower portions arrangedbetween the wider portion and the optical waveguide, the narrowerportions having a surface arranged facing the substrate, the widerportion having an upper surface arranged away from the substrate, andsubstantially planar side walls extending between the lower and uppersurfaces of the electrode, wherein the active electrode has roundedcorners between the side walls and the lower surface of the activeelectrode.
 13. An electro-optic modulator according to claim 12, theupper portion joining the two lower portions and extending over the twoparallel optical waveguides, the lower portions being arranged betweenthe upper portion and the substrate.
 14. The electro-optic modulator ofclaim 12, wherein the radius of curvature of each of the rounded cornersis between about 25% and 75% of the width of the contact area of theelectrode.
 15. The electro-optic modulator of claim 12, wherein theradius of curvature of each of the rounded corners is about half of thewidth of the contact area of the active electrode.
 16. The electro-opticmodulator of claim 12, wherein the width of the contact area of theactive electrode is less than half of the maximum width of the activeelectrode.
 17. The electro-optic modulator of claim 12, wherein theupper surface of the active electrode is convex.
 18. The electro-opticmodulator of claim 12, wherein the width of the each of the lowerportions is about eight microns.
 19. The electro-optic modulator ofclaim 12, wherein each of the two portions of the active electrodeoverlie one of the optical waveguides in an active region of themodulator.
 20. The electro-optic modulator according to claim 12,further comprising: at least one ground plane formed on the first faceof the substrate spaced apart from the active electrode.
 21. Theelectro-optic modulator according to claim 12, further comprising:ground planes formed on opposite sides of the active electrode, spacedapart from the active electrode.
 22. The electro-optic modulator ofclaim 13, wherein the at least one ground plane comprises a first groundplane on one side of the active electrode and a second ground planearranged on a side of the active electrode opposite the first groundplane.
 23. The electro-optic modulator of claim 12, further comprisingat least one groove in the substrate between the optical waveguides. 24.The electro-optic modulator of claim 12, further comprising at least onegroove in the substrate between the active electrode and the groundplane.
 25. The electro-optic modulator according to claim 12, whereinthe modulator is a Mach-Zehnder modulator.